It should be noted that reference to the prior art herein is not to be taken as an acknowledgement that such prior art constitutes common general knowledge in the art.
The Great Artesian Basin (GAB) extends across nearly two million square kilometers covering parts of Queensland, the Northern Territory, South Australia and New South Wales. It comprises 22 percent of the land in Australia. Due to a quirk of geology, the water held in the sponge-like rocks underground is under pressure. This means that when a bore is sunk, farmers often do not need a pump to bring water to the surface—it flows naturally. Like the Great Artesian Basin other countries have similar geological underground water reserves which provide substantial water supplies. The present invention is applicable to all countries having underground water reserves and unstable ground structures subject to subsidence.
Groundwater resources in the GAB and Bowen Basin support an extensive pastoral industry, inland population centres, mining activities, and other industries. There are many resources present in the basins—water, gas, oil and geothermal energy—and demand for these resources is increasing. As it is, the water is squashed between thick layers of sandstone into an interlinked network of aquifers. It “flows” through pores in the rock at the glacial pace of a few meters per year.
Beneath the layers of water lie some of the world's most extensive coal seams. Just as the sandstone aquifers contain water, so the coal seams contain methane dissolved in water. This methane or coal seam gas (CSG) is a form of natural gas that is extracted from the underground coal seams.
Protecting the water wealth of the GAB has created problems for the oil and coal seam gas industry. One of the most common activities causing subsidence is related to the withdrawal of ground fluids such as geothermal water or steam, ground water, and oil and gas. Each of these has the potential to cause maximum subsidence of the same order of magnitude to cause well integrity failure.
CSG or CBM (coal bed methane as it is termed in the United States of America) is trapped by groundwater pressure in the coal bed. CSG extraction occurs by drilling into the coal seam and lowering the groundwater pressure by pumping. Extracted coal seam waters are contaminated with brines and other compounds liberated from shale or coal seams during the drilling or fracking (back-flow) process. The latter often includes natural and induced petroleum compounds along with a certain amount of radionuclides (particularly radium) which has decayed from Uranium over millions of years. Various studies have shown these radiation levels to be highly elevated from the levels that would be acceptable under normal circumstances.
Other radioactive contaminants have included Thorium, Polonium, Radon gas, and some rarer daughter products. Once these radioactive contaminants reach the surface or underlying aquifers they create issues that would not exist if it had been possible to keep them deep underground.
With the increase in the importance, and hence value, of natural and CSG gas, gas leaks have become a very significant public issue. For environmental and other commercial reasons it is therefore desirable to find an affordable and safe way to control the migration of gas to the surface and underlying aquifers, even in wells that are no longer producing on a commercial scale.
Previous attempts by the gas production industry to address the problem have concentrated on variations of one-piece solutions. For example, concreting around the wellhead, attempting to reseal the annular gap around the damaged well stem casing, with Portland cement (the required minimum annular coating thickness around the well stem casing is only 13 mm thick (½ inch thick).
Accidents in the CSG industry are particularly dangerous, for many of the possible (in some cases inevitable) disastrous outcomes are such that they cannot be undone and hence cannot be remediated. Greenhouse gases and well fluids once released from the well stem whether on the surface or underground are unable to be recalled. Likewise the aquifers and soil once contaminated with gases and well fluids (including toxic salts) are rendered damaged beyond repair, and of little use or value for cropping irrigation or for animal and human consumption. Consequent serious human health impacts can have life-long and possible inter-generational repercussions.
It is therefore highly desirable for the protection of the environment that oil and gas well integrity is maintained and the potential for underground well leakage is kept to a minimum due to subsidence.
A major problem with the drilling and production of CSG is that of subsidence. One of the most common activities causing subsidence is related to the withdrawal of ground fluids such as geothermal water or steam, ground water, and oil and gas. Each of these has caused maximum subsidence of the ground or surface areas around oil and gas wells.
Subsidence around the well stem due to intersecting fault lines, either naturally occurring or induced by the collapse of the cap rock above or below the target coal seam area, caused by the compaction of the coal, induced when dewatering occurs, thus leaving an unsupported gap above the targeted coal bed.
The present system overcomes one or more of the above problems by providing a system of cement/concrete support columns and Bund walls, around the production well thereby significantly mitigating damage to environmental, social and personal safety risks bearing in mind that effective remediation options are limited just to re-pressurisation of the coal bed with water.